Among several factors which are associated with the folic acid deficiency of pregnancy are the increased folic acid requirement for fetal, placental and maternal tissue growth and an apparent, inadequate dietary intake. Evidence also exists for a decreased availability folic acid produced by the elevation of specific hormones in the maternal environment. A small, but significant number, of pregnant women in this country do not receive adequate nutritional supplementation during pregnancy for various reasons. These women are already "at risk" due to medical and other environmental factors, and there is a relatively high incidence of folic acid deficiency associated with megaloblastic anemia during their pregnancies. Because of the known disastrous effects on the fetus caused by severe folic acid deficiency, the relationship of the more common, borderline, folic acid deficiency to fetal growth will be investigated under controlled laboratory conditions. The experimental animal used for these studies will be the guinea pig, which is unique among small laboratory animals in exhibiting both a high requirement for folic acid and megaloblastic anemia due to folic acid deficiency. It is proposed that important biochemical and nutritional controversies relating to folic acid deficiency during pregnancy be resolved by utilizing this experimental animal since strictly controlled experiments using pregnant human subjects are both costly and not possible because of laws regulating human experimentation. Problems such as the effect of moderate folic acid deficiency on birth weight, neonatal mortality and fetal growth will be studied, as well as specific biochemical and hematological parameters of folic acid nutriture. These latter studies will include an assessment of the folic acid requirement during pregnancy in relation to the normal requirement and a comparison of maternal and fetal tissue stores of folic acid in guinea pigs fed opitmal or deficient amounts of folic acid.